1. Field of the Invention
The present invention relates to a refrigerating cycle device comprising a compressor capable of changing the discharge flow rate thereof, specifically, a variable displacement compressor capable of changing the discharge displacement, an electric compressor capable of controlling the rotation speed, etc., and more particularly, to a control of a discharge flow rate when the flow rate of a circulating refrigerant is low.
2. Description of the Related Art
Conventionally, in a dual-type air conditioner for a vehicle comprising a front seat air conditioning unit for air-conditioning a vehicle compartment front seat zone and a rear seat air conditioning unit for air-conditioning a vehicle compartment rear seat zone, evaporators are independently provided on the front seat side and the rear seat side, respectively, but a compressor and a condenser are shared.
The rear seat evaporator is arranged more distant from the compressor than the front seat evaporator and the refrigerant pipe path is long, therefore, the pressure loss in the refrigerant pipe path inevitably becomes large. Because of this, it is likely that the refrigerant flow rate is lower in the rear seat evaporator and the flow velocity of the refrigerant is reduced compared to that in the front seat evaporator. In addition to this, the low-pressure pipe of the rear seat evaporator is arranged normally under the floor of a vehicle and in many cases it is arranged at a position lower than the compressor suction pipe.
As a result, lubricant oil gathers in the rear seat evaporator or in the low-pressure pipe at its outlet and a problem is caused that the amount of oil returned to the compressor becomes low and the durability of the compressor is adversely affected. In order to solve this problem, there is a widely known refrigerating cycle device used for an air conditioner for a vehicle, in which the operation of a fixed displacement compressor is forcedly controlled so that it operates intermittently or the lack of the amount of oil returned to the compressor is solved by forcedly changing the discharge displacement of the variable displacement compressor (for example, refer to Patent document 1).
In addition to Patent document 1 described above, based on the research and examination by the applicants of the present invention, it has been found that the lack of the amount of oil returned to the compressor becomes more conspicuous when the flow rate of the circulating refrigerant in the refrigerating cycle falls below a predetermined threshold value determined for each type of cycle and that the variations in the temperature distribution of the discharged air temperature of the evaporator become large when the flow rate of the circulating refrigerant falls below the predetermined threshold value.
However, according to Patent document 1 described above, the continuous operation time of the compressor after it is started is measured and when the continuous operation time of the compressor reaches a predetermined time, only the following control is carried out, that is, the operation of the fixed displacement compressor is controlled so that it operates intermittently or the operation of the compressor is controlled by forcedly changing the discharge displacement of the variable displacement compressor.
In other words, no control directly relating to the fall in the flow rate of the circulating refrigerant in the refrigerating cycle is carried out. Therefore, with the intention of preventing the lack of oil returned to the compressor or of improving the variations in the discharged air temperature, the applicants of the present invention have filed an application for a device characterized in that, when the flow rate of the circulating refrigerant is higher than a predetermined threshold value, the discharge flow rate of the compressor is controlled continuously in accordance with the degree of cooling of the evaporator and when the flow rate of the circulating refrigerant is lower than the predetermined threshold value, the discharge flow rate of the compressor is changed intermittently between an intermediate flow rate near the predetermined threshold value and a low flow rate lower than the predetermined threshold value (for example, refer to Japanese Patent Application No. 2003-60037).
[Patent document 1] Japanese Unexamined Patent Publication (Kokai) No. 2000-283576)
However, based on-subsequent research and an examination by the inventors of the present invention, it has been found that, in the device filed for the above-mentioned application, after the ON/OFF control of the device in which the flow rate is changed intermittently is carried out at a low flow rate near the predetermined threshold value, if the continuous control is carried out at the low flow rate near the predetermined threshold value, there appears a phenomenon that the evaporator is in a state in which the evaporation temperature overshoots immediately after the compressor is turned ON while the compressor is operated intermittently and the temperature of the evaporator falls rapidly when the control is switched to the continuous control.
In other words, as the control current for operating the compressor in order to change the discharge flow rate is fed back based on the evaporation temperature, such a measure is taken that when the evaporation temperature overshoots, the control current is increased and when the evaporation temperature falls, the control current is reduced, however, it has been found that if the evaporation temperature falls rapidly, it is difficult to control the control current so as to follow the change of the evaporation temperature.
Further, it has been found that, if the control current is difficult to control, the evaporation temperature may continue to fall and reach the frosting region or the control current may fall below the predetermined threshold value and the control is changed to a control for changing the operation intermittently, resulting in the occurrence of pressure hunting or variations in the evaporator discharged air temperature.